package com.mogujie.jessica.util;


/**
 * An iterator to iterate over set bits in an OpenBitSet. This is faster than
 * nextSetBit() for iterating over the complete set of bits, especially when the
 * density of the bits set is high.
 */
public class OpenBitSetIterator extends DocIdSetIterator
{

    // The General Idea: instead of having an array per byte that has
    // the offsets of the next set bit, that array could be
    // packed inside a 32 bit integer (8 4 bit numbers). That
    // should be faster than accessing an array for each index, and
    // the total array size is kept smaller (256*sizeof(int))=1K
    protected final static int[] bitlist = { 0x0, 0x1, 0x2, 0x21, 0x3, 0x31, 0x32, 0x321, 0x4, 0x41, 0x42, 0x421, 0x43, 0x431, 0x432, 0x4321, 0x5, 0x51, 0x52, 0x521, 0x53, 0x531, 0x532, 0x5321, 0x54, 0x541, 0x542, 0x5421, 0x543, 0x5431, 0x5432, 0x54321, 0x6, 0x61, 0x62, 0x621, 0x63, 0x631, 0x632, 0x6321, 0x64, 0x641, 0x642, 0x6421, 0x643, 0x6431, 0x6432, 0x64321, 0x65, 0x651, 0x652, 0x6521, 0x653, 0x6531, 0x6532, 0x65321, 0x654, 0x6541, 0x6542, 0x65421, 0x6543, 0x65431, 0x65432, 0x654321, 0x7, 0x71, 0x72, 0x721, 0x73, 0x731, 0x732, 0x7321, 0x74, 0x741, 0x742, 0x7421, 0x743, 0x7431, 0x7432, 0x74321, 0x75, 0x751, 0x752, 0x7521, 0x753, 0x7531, 0x7532, 0x75321, 0x754, 0x7541, 0x7542, 0x75421, 0x7543, 0x75431, 0x75432, 0x754321, 0x76, 0x761, 0x762, 0x7621, 0x763, 0x7631, 0x7632, 0x76321, 0x764, 0x7641, 0x7642, 0x76421, 0x7643, 0x76431, 0x76432, 0x764321, 0x765, 0x7651, 0x7652, 0x76521, 0x7653, 0x76531, 0x76532, 0x765321, 0x7654, 0x76541, 0x76542, 0x765421, 0x76543, 0x765431, 0x765432, 0x7654321, 0x8, 0x81, 0x82, 0x821, 0x83, 0x831, 0x832, 0x8321, 0x84, 0x841, 0x842, 0x8421, 0x843, 0x8431, 0x8432, 0x84321, 0x85, 0x851, 0x852, 0x8521, 0x853, 0x8531, 0x8532, 0x85321, 0x854, 0x8541, 0x8542, 0x85421, 0x8543, 0x85431, 0x85432, 0x854321, 0x86, 0x861, 0x862, 0x8621, 0x863, 0x8631, 0x8632, 0x86321, 0x864, 0x8641, 0x8642, 0x86421, 0x8643, 0x86431, 0x86432, 0x864321, 0x865, 0x8651, 0x8652, 0x86521, 0x8653, 0x86531, 0x86532, 0x865321, 0x8654, 0x86541, 0x86542, 0x865421, 0x86543, 0x865431, 0x865432, 0x8654321, 0x87, 0x871, 0x872, 0x8721, 0x873, 0x8731, 0x8732, 0x87321, 0x874, 0x8741, 0x8742, 0x87421, 0x8743, 0x87431, 0x87432, 0x874321, 0x875, 0x8751, 0x8752, 0x87521, 0x8753, 0x87531, 0x87532, 0x875321, 0x8754, 0x87541, 0x87542, 0x875421, 0x87543, 0x875431, 0x875432, 0x8754321, 0x876, 0x8761, 0x8762, 0x87621, 0x8763, 0x87631, 0x87632, 0x876321, 0x8764, 0x87641, 0x87642, 0x876421, 0x87643, 0x876431, 0x876432, 0x8764321, 0x8765, 0x87651, 0x87652, 0x876521, 0x87653, 0x876531, 0x876532, 0x8765321, 0x87654, 0x876541, 0x876542, 0x8765421, 0x876543, 0x8765431, 0x8765432, 0x87654321 };
    /*****
     * the python code that generated bitlist def bits2int(val): arr=0 for shift
     * in range(8,0,-1): if val & 0x80: arr = (arr << 4) | shift val = val << 1
     * return arr
     * 
     * def int_table(): tbl = [ hex(bits2int(val)).strip('L') for val in
     * range(256) ] return ','.join(tbl)
     ******/

    // hmmm, what about an iterator that finds zeros though,
    // or a reverse iterator... should they be separate classes
    // for efficiency, or have a common root interface? (or
    // maybe both? could ask for a SetBitsIterator, etc...

    private final long[] arr;
    private final int words;
    private int i = -1;
    private long word;
    private int wordShift;
    private int indexArray;
    private int curDocId = -1;

    public OpenBitSetIterator(OpenBitSet obs)
    {
        this(obs.getBits(), obs.getNumWords());
    }

    public OpenBitSetIterator(long[] bits, int numWords)
    {
        arr = bits;
        words = numWords;
    }

    // 64 bit shifts
    private void shift()
    {
        if ((int) word == 0)
        {
            wordShift += 32;
            word = word >>> 32;
        }
        if ((word & 0x0000FFFF) == 0)
        {
            wordShift += 16;
            word >>>= 16;
        }
        if ((word & 0x000000FF) == 0)
        {
            wordShift += 8;
            word >>>= 8;
        }
        indexArray = bitlist[(int) word & 0xff];
    }

    /*****
     * alternate shift implementations // 32 bit shifts, but a long shift needed
     * at the end private void shift2() { int y = (int)word; if (y==0)
     * {wordShift +=32; y = (int)(word >>>32); } if ((y & 0x0000FFFF) == 0) {
     * wordShift +=16; y>>>=16; } if ((y & 0x000000FF) == 0) { wordShift +=8;
     * y>>>=8; } indexArray = bitlist[y & 0xff]; word >>>= (wordShift +1); }
     * 
     * private void shift3() { int lower = (int)word; int lowByte = lower &
     * 0xff; if (lowByte != 0) { indexArray=bitlist[lowByte]; return; } shift();
     * }
     ******/

    @Override
    public int nextDoc()
    {
        if (indexArray == 0)
        {
            if (word != 0)
            {
                word >>>= 8;
                wordShift += 8;
            }

            while (word == 0)
            {
                if (++i >= words)
                {
                    return curDocId = NO_MORE_DOCS;
                }
                word = arr[i];
                wordShift = -1; // loop invariant code motion should move this
            }

            // after the first time, should I go with a linear search, or
            // stick with the binary search in shift?
            shift();
        }

        int bitIndex = (indexArray & 0x0f) + wordShift;
        indexArray >>>= 4;
        // should i<<6 be cached as a separate variable?
        // it would only save one cycle in the best circumstances.
        return curDocId = (i << 6) + bitIndex;
    }

    @Override
    public int advance(int target)
    {
        indexArray = 0;
        i = target >> 6;
        if (i >= words)
        {
            word = 0; // setup so next() will also return -1
            return curDocId = NO_MORE_DOCS;
        }
        wordShift = target & 0x3f;
        word = arr[i] >>> wordShift;
        if (word != 0)
        {
            wordShift--; // compensate for 1 based arrIndex
        } else
        {
            while (word == 0)
            {
                if (++i >= words)
                {
                    return curDocId = NO_MORE_DOCS;
                }
                word = arr[i];
            }
            wordShift = -1;
        }

        shift();

        int bitIndex = (indexArray & 0x0f) + wordShift;
        indexArray >>>= 4;
        // should i<<6 be cached as a separate variable?
        // it would only save one cycle in the best circumstances.
        return curDocId = (i << 6) + bitIndex;
    }

    @Override
    public int docID()
    {
        return curDocId;
    }

}
